Optical glass

Abstract

An optical glass comprising, by mass %, 12 to 40% of SiO 2 , 15% or more but less than 42% of Nb 2 O 5 , 2% or more but less than 18% of TiO 2 , (provided that Nb 2 O 5 /TiO 2 is over 0.6), 0.1 to 20% of Li 2 O, 0.1 to 15% of Na 2 O, and 0.1 to 25% of K 2 O, and having an Abbe's number νd of 20 to 30, a ΔPg,F of 0.016 or less and a liquidus temperature of 1,200° C. or lower.

The invention claimed is: 
     
       1. An optical glass comprising, by mass %,
 12 to 40% of SiO 2 , 
 15% or more but less than 42% of Nb 2 O 5 , 
 2% or more but less than 12% of TiO 2 , 
 (provided that Nb 2 O 5 /TiO 2  is equal to or more than 3.49), 
 0.1 to 20% of Li 2 O, 
 0.1 to 15% of Na 2 O, and 
 0.1 to 25% of K 2 O, 
 the optical glass having an Abbe's number νd of 20 to 30, a refractive index nd of 1.87 to 1.90, ΔPg,F of 0.016 or less and a liquidus temperature of 1,200° C. or lower. 
 
     
     
       2. The optical glass as recited in  claim 1 , which contains, as optional components,
 0 to 10% of B 2 O 3 , 
 0 to 20% of ZrO 2 , 
 0 to 20% of WO 3 , 
 0 to 13% of CaO, 
 0 to 13% of SrO, 
 0 to 20% of BaO, 
 (provided that the total content of CaO, SrO and BaO is 0 to 25%), 
 0 to 13% of ZnO, 
 0 to 3% of La 2 O 3 , 
 0 to 3% of Gd 2 O 3 , 
 0 to 3% of Y 2 O 3 , 
 0 to 3% of Yb 2 O 3 , 
 0 to 10% of Ta 2 O 5 , 
 0 to 3% of GeO 2 , 
 0 to 10% of Bi 2 O 3 , and 
 0 to 10% of Al 2 O 3 , 
 the total content of Nb 2 O 5  and TiO 2  being 30 to 60%, the content of K 2 O being 0.1 to 15%, and the total content of Li 2 O, Na 2 O and K 2 O being 1 to 25%. 
 
     
     
       3. The optical glass as recited in  claim 1 , which contains Sb 2 O 3  in an amount of 0 to 2% and SnO 2  in an amount of 0 to 2%, these amounts being based on a glass composition excluding Sb 2 O 3  and SnO 2 . 
     
     
       4. The optical glass as recited in  claim 1 , which has a partial dispersion ratio, Pg,F, of 0.580 to 0.620. 
     
     
       5. The optical glass as recited in  claim 1 , which has a glass transition temperature of less than 600° C. 
     
     
       6. A press-molding glass material formed of the optical glass recited in  claim 1 . 
     
     
       7. A process for producing an optical element blank, which comprises softening the glass material recited in  claim 6  under heat and press-molding the glass material. 
     
     
       8. A process for producing an optical element, which comprises grinding and polishing the optical element blank produced by the process recited in  claim 7 . 
     
     
       9. A process for producing an optical element, which comprises heating the press-molding glass material recited in  claim 6  and precision press-molding it with a press mold. 
     
     
       10. The process for producing an optical element as recited in  claim 9 , which comprises introducing the glass material into the press mold and heating said mold and the glass material together. 
     
     
       11. A process for producing an optical element as recited in  claim 9 , which comprises heating the glass material and introducing the glass material to a pre-heated press mold to carry out the precision press-molding. 
     
     
       12. An optical element formed of the optical glass recited in  claim 1 . 
     
     
       13. A process for producing an optical element blank, which comprises supplying a molten glass to a press mold and press-molding the molten glass,
 wherein glass raw materials prepared so as to obtain the optical glass recited in  claim 1  are melted under heat and the resultant molten glass is press-molded.